The invention relates to rubber mixtures, to their preparation and to their use.
It is known that hydrolysable sulphur-containing organosilicon compounds are capable of reacting with fillers containing hydroxy groups, e.g. natural and synthetic silicates, carbonates, glasses and metal oxides. They are used here for surface modification and to promote adhesion. The rubber-processing industry uses them as coupling agents between the reinforcing filler and the polymer used (DE2141159, DE2212239, DE19544469A1, U.S. Pat. No. 3,978,103, U.S. Pat. No. 4,048,206, EP784072A1).
It is moreover known that the use of commercially available silane coupling agents having three alkyloxy substituents on the silicon atom (DE 22 55 577), e.g. bis[3-triethoxysilylpropyl]tetrasulphide or bis[3-triethoxysilylpropyl]disulphide leads to liberation of considerable amounts of alcohol during and after coupling to the filler. Since trimethoxy- and triethoxy-substituted silanes are generally used, the corresponding alcohols methanol and ethanol are liberated in the course of the application (e.g. page 18 in Berkemeier, D.; Hader, W.; Rinker, M.; Heiss, G., Mixing of silica compounds from the viewpoint of a manufacturer of internal mixers, Gummi, Fasern, Kunststoffe (2001), 54(1), 17-22).
It is moreover known that methoxy-substituted silanes have higher hydrolysis-activity than ethoxy-substituted silanes. Ethoxy-substituted silanes have higher hydrolysis-activity than longer-chain or branched alkoxy-substituted silanes having more than 2 carbon atoms (E. R. Pohl, F. D. Osterholtz J. Adhesion Sci. Technology 6(1) 1992, 127-149). They are therefore capable of faster coupling to the filler, and use of methoxy- and ethoxy-substituted silanes has therefore hitherto been essential for economic reasons.
A considerable disadvantage in the use of known alkoxy silane coupling agents, specifically of bis(trialkoxysilylalkyl)polysulphide coupling agents, is the liberation of stoichiometric amounts of volatile alcohols, such as methanol and ethanol, into the environment during and after coupling of the alkoxysilane to the filler.
Another disadvantage in the use of bis(triethoxysilylpropyl)polysulphide coupling agents is the limitation on mixing temperature for rubber mixtures to the temperature range from 140 to 165° C. (H. D Luginsland, “A Review on the chemistry and the reinforcement of the silica-silane filler system for rubber applications, Shaker Verlag, Aachen 2002, page 34).
U.S. Pat. No. 6,433,206, DE 25 42 534 C3, DE 24 05 758 C3 and DE 27 12 866 A1 disclose polysulphide-containing silatranes. They can be used as reinforcing additives in rubber mixtures comprising silicatic fillers.